The use of industrial processing equipment, particularly equipment used for aqueous processes, generally results in the buildup of water insoluble deposits, especially at elevated temperatures. These deposits require periodic removal from the inner surfaces of tanks, conduits and the like without disassembling the components thereof. Acid solutions have been used successfully to remove these deposits, e.g. mill scale and fly ash, in industries such as chemical and petroleum plants, paper mills, utility companies and the like.
In the cleaning of such metal surfaces using aqueous solutions of acids wherein the cleaning of the metal surfaces includes the removal of acid soluble, water-insoluble deposits thereon, such as mmetal oxide scale, an inhibitor is often used in the cleaning solution to reduce acid attack on the metal surface. When the term "scale" is used hereinafter, it is understood to include other acid soluble, water-insoluble deposits as well as metal oxide scale. Both inorganic and organic acid baths are currently being used commercially for the removal of scale.
Among the acids generally used for industrial cleaning the inorganic acids, particularly, the mineral acids such as hydrochloric, sulfuric, nitric, and phosphoric acid are most frequently used, though others are also used depending upon the particular needs. In still other cases, organic acids including formic acid, citric acid, mixtures of hydroxy acetic and formic acids, and acetic acid and other organic acids, such as oxalic acid, tartaric acid and alkylene polyamine carboxylic acids as well as water soluble salts and mixes of acids and salts are used.
One of the mineral acids that has been used successfully in the removal of scale is hydrofluoric acid. Compositions of hydrofluoric acid containing an inhibitor are disclosed in U.S. Pat. No. 3,992,313 and No. 4,104,303 to Anderson et al. The compositions disclosed in the Anderson patents are disclosed for use with ferrous metals and with hydrofluoric acid as the acid cleaning agent. The Anderson compositions contain an inhibitor which comprises a Mannich base and thiourea.
Inhibitors when stored and shipped for end use in forming the acid cleaning baths, may gel and/or solidify at low storage and shipping temperatures and the components thereof may separate or precipitate on warming. When concentrates of inhibitors with organic acids were formed, such concentrates also exhibited precipitation problems due to inherently low solubility and/or stability of the inhibitors in the concentrates.
In the field of acid cleaning of scale from metal surfaces there is needed a bath composition that will clean metals in addition to those that are ferrous based and in which the bath will operate regardless of the acid chosen i.e. will be operable in either a bath based on an inorganic acid or on an organic acid. In addition, in order to be commercially feasible, the bath must also inhibit acid attack on the metal surfaces. A further need in this field is for a concentrated composition which contains the inhibitor plus surfactant which can be added to an acid cleaning bath and which is stable in the concentrated form. An additional need is for the concentrated composition that contains all of the ingredients required to form an organic acid-based cleaning bath upon dilution thereof with water, and which will be stable in the concentrated form.